Microphone arrays are well known in the field of acoustics. By combining the outputs of several microphones in an array electronically, a directional sound pickup pattern can be achieved. This means that sound arriving from a small range of directions is emphasized while sound coming from other directions is attenuated. Such a capability is useful in areas such as telephony, teleconferencing, video conferencing, hearing aids, and the detection of sound sources outdoors. However, practical considerations mitigate against physically large arrays. It is therefore desirable to obtain as much acoustical directionality out of as small an array as possible.
Normally, reduced array size can be achieved by utilizing superdirective approaches in the combining of microphone signals rather than the more conventional delay and sum beamforming usually used in array signal processing. While superdirective approaches do work, the resulting array designs can be very sensitive to the effects of microphone self noise and errors in matching microphone amplitude and phase responses.
A few approaches have been attempted in the field to solve the above problem. Elko, in U.S. Pat. No. 5,742,693 considers the improved directionality obtained by placing a first order microphone near a plane baffle, giving an effective second order system. Unfortunately, the system described is unwieldy. Elko notes that when choosing baffle dimensions, the largest possible baffle is most desirable. Also, to achieve a second order response, Elko notes that the baffle size should be in the order of at least one-half a wavelength of the desired signal. These requirements render Elko unsuitable for applications requiring physically small arrays.
Bartlett et al, in U.S. Pat. No. 5,539,834 discloses achieving a second order effect from a first order microphone. Bartlett achieves a performance enhancement by using a reflected signal from a plane baffle. However, Bartlett does not achieve the desired directivity required in some applications. While Bartlett would be useful as a microphone in a cellular telephone handset, it cannot be readily adapted for applications such as handsfree telephony or teleconferencing in which high directionality is desirable.
Another approach, taken by Kuhn in U.S. Pat. No. 5,592,441, uses forty-two transducers on the vertices of a regular geodesic two frequency icosahedron. While Kuhn may produce the desired directionality, it is clear that Kuhn is quite complex and impractical for the uses envisioned above.
Another patent, issued to Elko et al, U.S. Pat. No. 4,802,227, addresses signal processing aspects of microphone arrays. Elko et al however, utilizes costly signal processing means to reduce noise. The signal processing capabilities required to keep adaptively calculating the required real-time analysis can be prohibitive.
A further patent, issued to Gorike, U.S. Pat. No. 4,904,078 uses directional microphones in eyeglasses to assist persons with a hearing disability receiving aural signals. The directional microphones, however, do not allow for a changing directionality as to the source of the sound.
The use of diffraction can effectively increase the aperture size and the directionality of a microphone array. Thus, diffractive effects and the proper design of diffractive surfaces can provide large aperture sizes and improved directivity with relatively small arrays. When implemented using superdirective beamforming, the resulting array is less sensitive to microphone self noise and errors in matching microphone amplitude and phase responses. A simple example of how a diffracting object can improve the directional performance of a system is provided by the human head and ears. The typical separation between the ears of a human is 15 cm. Measurements of two-ear correlation functions in reverberant rooms show that the effective separation is more than double this, about 30 cm, which is the ear separation around a half-circumference of the head.
Academic papers have recently suggested that diffracting structures can be used with microphone arrays. An oral paper by Kawahara and Fukudome, (“Superdirectivity design for a sphere-baffled microphone”, J. Acoust. Soc. Am. 130, 2897, 1998), suggests that a sphere can be used to advantage in beamforming. A six-microphone configuration mounted on a sphere was discussed by Elko and Pong, (“A steerable and variable 1st order differential microphone array”, Intl. Conf. On Acoustics, Speech and Signal Processing, 1997), noting that the presence of the sphere acted to increase the effective separation of the microphones. However, these two publications only consider the case of a rigid intervening sphere.
What is therefore required is a directional microphone array which is relatively inexpensive, small, and can be easily adapted for electro acoustic applications such as teleconferencing and hands free telephony.